Bone loss is a worldwide problem resulting in increased risk of fracture and tooth loss27. Osteoblasts are responsible for bone synthesis; therefore, treatments promoting osteoblast differentiation and/or activity would result in increased bone formation28. The regulation of DNA accessibility is a key mechanism controlling gene expression and cellular differentiation. BAF (BRG1 Associated Factor) mediated Chromatin remodeling increases DNA accessibility by sliding or ejecting nucleosomes1,2,14. This process can occur in a cell type or stage specific manner based on the composition of the BAF complex4. In many tissue types, a unique combination of BAF subunits has been identified to be responsible for the maintenance or differentiation of that cell type4,5,6,7. To date, an osteoblast specific BAF chromatin remodeling complex has yet to be identified. It has been recently found in our studies that BAF45A, a potential member of the BAF complex, plays an essential role in osteoblast differentiation and function. The purpose of this study is to address our central hypothesis that that BAF45A is critical to remodel and activate chromatin at osteoblast specific genes promoting the synthesis and maintenance of bone. In order to address this hypothesis the study has been divided into two specific aims. 1) Define the phenotype of BAF45A osteoblast-specific knockout mice: To accomplish this aim, we have developed an osteoblast-specific knockout of Baf45a. This is done by crossing a Baf45a floxed allele mouse with an osteoblast specific osteocalcin-Cre mouse. The physical phenotype of this mouse will be analyzed by whole skeletal staining, microCT, histology/histomorphometry, double calcein labeling, and a three-point bend test. The molecular properties will be assessed using real time PCR for osteoblastic gene expression analysis, and chromatin-immunoprecipitation in order to understand the corresponding chromatin state at osteoblast specific genes. 2) Identify osteoblast differentiation stage-specific BAF complex composition: To uncover the composition of the osteoblast specific BAF complex, a biotin proximity ligation technique will be employed. A promiscuous biotin ligase will be linked with BAF45A and proteins in close proximity will be biotinylated, pulled-down, and identified by mass spectrometry. This will be confirmed by co- immunoprecipitation. Understanding the physiological role and molecular mechanism of Baf45a as well as the composition of BAF in osteoblasts will reveal novel insights into osteoblast development and function. This may ultimately result in the development of treatments targeted to increase osteoblast differentiation and activity to increase bone mass.